In the medical field, various types of body-implantable electrical leads are known and used. Particularly in the field of cardiac pulse generators, the use of implanted pacing and/or sensing leads is very common. Implantable cardiac pulse generators are typically implanted either in the region of a patient's thorax, for example under the skin near the patient's left or right clavicle, or in the patient's abdomen. A pacemaker lead, having a proximal end coupled to the pulse generator and a distal end that is in electrical contact with the patient's heart muscle, functions to convey electrical cardiac signals to sensing circuitry associated with the pulse generator, and/or to convey electrical stimulating pulses (e.g., pacing pulses) to the cardiac muscle from the pulse generator.
For endocardial leads, where the electrode tip of the lead is firmly lodged in or against endocardial tissue and chronically secured thereto in some fashion, the lead assembly is typically introduced through a body vessel, such as a vein, into one or more cardiac chambers. The conductor within the lead is protected by a biocompatible insulating material. A stylet may be provided for improving the maneuverability of the lead when inserted into, and guided through, the veins for positioning in the desired heart chamber. Examples of conventional endocardial leads in the prior art include: U.S. Pat. No. 3,348,548 to Chardack; U.S. Pat. No. 3,754,555 to Schmitt; U.S. Pat. No. 3,814,104 to Irnich et al.; U.S. Pat. No. 3,844,292 to Bolduc; and U.S. Pat. No. 3,974,834 to Kane.
Among the desirable attributes of endocardial leads are: minimal lead diameter; secure fixation of the electrode to cardiac tissue to prevent electrode dislodgement; implantation control with minimal damage to the vein, heart valves, cardiac tissue, or other tissue with which the electrode and lead come into contact; control of the fixation process to avoid excessive damage of tissue by the fixation mechanism; minimal electrical threshold; and maximal pacing impedance.
Another desirable attribute relates to the ability to bring a lead's electrode into temporary contact with a potential fixation site during lead introduction, so the electrical suitability of the selected site can be assessed prior to permanent (chronic) fixation of the electrode at the selected site. This function is commonly referred to as "mapping."
A lead's mappability is determined primarily by the configuration of the electrode assembly during transvenous introduction of the lead. For example, if a lead's electrode is required to be maintained in some type of retracted condition during lead introduction (as for leads whose sharp helical coil serves both as the fixation mechanism and the electrode), that lead's mappability will be poor since the electrode must be extended out from its retracted position before it can make electrical contact at the proposed implant site. For "screw-in" leads, often the suitability of a proposed fixation site cannot be fully assessed until the lead is screwed in to the tissue. In this case, if a site is found to be unsuitable, the lead must be unscrewed and a new site selected before the assessment process can proceed. This is undesirable because it increases the time necessary to perform a lead introduction procedure and also can result in tissue damage arising out of affixing the electrode at a site subsequently found to be unsuitable.